Electronic spin precession and interferometry from spin-orbital entanglement in a double quantum dot

A double quantum dot inserted in parallel between two metallic leads allows to entangle the electron spin with the orbital (dot index) degree of freedom. An Aharonov-Bohm orbital phase can then be transferred to the spinor wavefunction, providing a geometrical control of the spin precession around a fixed magnetic field. A fully coherent behaviour is obtained in a mixed orbital/spin Kondo regime. Evidence for the spin precession can be obtained, either using spin-polarized metallic leads or by placing the double dot in one branch of a metallic loop.Comment: Final versio

Collective pinning of the vortex lattice by columnar defects in layered superconductors

The mixed phase of layered superconductors with no magnetic screening is studied through a partial duality analysis of the corresponding frustrated XY model in the presence of random columnar pins. A small fraction of pinned vortex lines is assumed. Thermally induced plastic creep of the vortex lattice within isolated layers results in an intermediate Bose glass phase that exhibits weak superconductivity across layers in the limit of weak Josephson coupling. The correlation volume of the vortex lattice is estimated in the strongly-coupled Bose-glass regime at lower temperature. In the absence of additional point pins, no peak effect in the critical current density is predicted to occur on this basis as a function of the Josephson coupling. Also, the phase transition observed recently inside of the vortex-liquid phase of high-temperature superconductors pierced by sparse columnar defects is argued to be a sign of dimensional cross-over.Comment: 16 pages, 1 figure, account of transition to ``nanoliquid'' in BSCCO, to appear in PR

Resonant Tunneling through Multi-Level and Double Quantum Dots

We study resonant tunneling through quantum-dot systems in the presence of strong Coulomb repulsion and coupling to the metallic leads. Motivated by recent experiments we concentrate on (i) a single dot with two energy levels and (ii) a double dot with one level in each dot. Each level is twofold spin-degenerate. Depending on the level spacing these systems are physical realizations of different Kondo-type models. Using a real-time diagrammatic formulation we evaluate the spectral density and the non-linear conductance. The latter shows a novel triple-peak resonant structure.Comment: 4 pages, ReVTeX, 4 Postscript figure

Zero-bias Anomaly of Tunneling into the Edge of a 2D Electron System

We investigate the electron tunneling into the edge of a clean weakly interacting two-dimensional electron gas. It is shown that the corresponding differential conductance $G(V)$ has a cusp at zero bias, and is characterized by a universal slope $|dG/dV|$ at $V=0$. This singularity originates from the electron scattering on the Friedel oscillation caused by the boundary of the system.Comment: 10 pages, uuencoded compressed Postscript file, to appear in Phys. Rev. B (Rapid Communications

Critical conductance of a one-dimensional doped Mott insulator

We consider the two-terminal conductance of a one-dimensional Mott insulator undergoing the commensurate-incommensurate quantum phase transition to a conducting state. We treat the leads as Luttinger liquids. At a specific value of compressibility of the leads, corresponding to the Luther-Emery point, the conductance can be described in terms of the free propagation of non-interacting fermions with charge e/\sqrt{2}. At that point, the temperature dependence of the conductance across the quantum phase transition is described by a Fermi function. The deviation from the Luther-Emery point in the leads changes the temperature dependence qualitatively. In the metallic state, the low-temperature conductance is determined by the properties of the leads, and is described by the conventional Luttinger liquid theory. In the insulating state, conductance occurs via activation of e/\sqrt{2} charges, and is independent of the Luttinger liquid compressibility.Comment: 13 pages, 3 figures. Published versio

Kondo Shuttling in Nanoelectromechanical Single-Electron Transistor

We investigate theoretically a mechanically assisted Kondo effect and electric charge shuttling in nanoelectromechanical single-electron transistor (NEM-SET). It is shown that the mechanical motion of the central island (a small metallic particle) with the spin results in the time dependent tunneling width which leads to effective increase of the Kondo temperature. The time-dependent oscillating Kondo temperature T_K(t) changes the scaling behavior of the differential conductance resulting in the suppression of transport in a strong coupling- and its enhancement in a weak coupling regimes. The conditions for fine-tuning of the Abrikosov-Suhl resonance and possible experimental realization of the Kondo shuttling are discussed.Comment: 4 pages, 2 eps figure

Theory of the Franck-Condon blockade regime

Strong coupling of electronic and vibrational degrees of freedom entails a low-bias suppression of the current through single-molecule devices, termed Franck-Condon blockade. In the limit of slow vibrational relaxation, transport in the Franck-Condon-blockade regime proceeds via avalanches of large numbers of electrons, which are interrupted by long waiting times without electron transfer. The avalanches consist of smaller avalanches, leading to a self-similar hierarchy which terminates once the number of transferred electrons per avalanche becomes of the order of unity. Experimental signatures of self-similar avalanche transport are strongly enhanced current (shot) noise, as expressed by giant Fano factors, and a power-law noise spectrum. We develop a theory of the Franck-Condon-blockade regime with particular emphasis on effects of electron cotunneling through highly excited vibrational states. As opposed to the exponential suppression of sequential tunneling rates for low-lying vibrational states, cotunneling rates suffer only a power-law suppression. This leads to a regime where cotunneling dominates the current for any gate voltage. Including cotunneling within a rate-equation approach to transport, we find that both the Franck-Condon blockade and self-similar avalanche transport remain intact in this regime. We predict that cotunneling leads to absorption-induced vibrational sidebands in the Coulomb-blockaded regime as well as intrinsic telegraph noise near the charge degeneracy point.Comment: 20 pages, 10 figures; minor changes, version published in Phys. Rev.

Coulomb Blockade with Dispersive Interfaces

What quantity controls the Coulomb blockade oscillations if the dot--lead conductance is essentially frequency--dependent ? We argue that it is the ac dissipative conductance at the frequency given by the effective charging energy. The latter may be very different from the bare charging energy due to the interface--induced capacitance (or inductance). These observations are supported by a number of examples, considered from the weak and strong coupling (perturbation theory vs. instanton calculus) perspectives.Comment: 4 page

On the applicability of the equations-of-motion technique for quantum dots

The equations-of-motion (EOM) hierarchy satisfied by the Green functions of a quantum dot embedded in an external mesoscopic network is considered within a high-order decoupling approximation scheme. Exact analytic solutions of the resulting coupled integral equations are presented in several limits. In particular, it is found that at the particle-hole symmetric point the EOM Green function is temperature-independent due to a discontinuous change in the imaginary part of the interacting self-energy. However, this imaginary part obeys the Fermi liquid unitarity requirement away from this special point, at zero temperature. Results for the occupation numbers, the density of states and the local spin susceptibility are compared with exact Fermi liquid relations and the Bethe ansatz solution. The approximation is found to be very accurate far from the Kondo regime. In contrast, the description of the Kondo effect is valid on a qualitative level only. In particular, we find that the Friedel sum rule is considerably violated, up to 30%, and the spin susceptibility is underestimated. We show that the widely-used simplified version of the EOM method, which does not account fully for the correlations on the network, fails to produce the Kondo correlations even qualitatively.Comment: 16 pages, 5 figure

A minimal model of quantized conductance in interacting ballistic quantum wires

We review what we consider to be the minimal model of quantized conductance in a finite interacting quantum wire. Our approach utilizes the simplicity of the equation of motion description to both deal with general spatially dependent interactions and finite wire geometry. We emphasize the role of two different kinds of boundary conditions, one associated with local "chemical" equilibrium in the sense of Landauer, the other associated with screening in the proximity of the Fermi liquid metallic leads. The relation of our analysis to other approaches to this problem is clarified. We then use our formalism to derive a Drude type expression for the low frequency AC-conductance of the finite wire with general interaction profile.Comment: 6 pages, 2 figures; extended discussion, references adde